We are interested in the nature and mechanisms of cell interactions in the myxobacterium, Myxococcus xanthus. These interactions occur throughout the growth and complex life cycle of this organism. Some of these interactions seem to involve cell contact and we are interested in the nature of these contact-mediated interactions. We intend to use monoclonal antibodies (MCA) against cell-surface antigens as a probe for identifying and characterizing the cell-surface signals and their receptors. We will isolate as large a number of MCA against different cell-surface antigens as feasible; these will be checked for identity by Western blot against electrophoretically separated cell-surface antigens. The MCA will be used to block development by intact cells, as a selection technique for isolating cell-surface antigen mutants, by affinity chromatography to isolate the cell-surface antigens whose blockage interferes with development, and as a cell-sorting technique to detect population differentiation during development. Eventually, this may lead to isolation of the corresponding receptor sites, characterization of the signal-receptor interactions, and the nature of the signal transduction. The existence of a system for genetic analysis in M. xanthus makes it feasible to think of an eventual genetic analysis of these specifically defined interactions.